Conventionally, Magnetic Resonance Imaging (MRI) has been used to visualize characteristics of soft tissue of the human body and to diagnose diseased tissue.
Pre-polarized Magnetic Resonance Imaging (PMRI) has been promoted as a method of constructing low-cost imaging systems. The basic principle of the PMRI method is to immerse or otherwise subject an object of interest in a transient magnetic field. The purpose of said immersion is to polarize and/or align spins in the object along the direction of the transient magnetic field. For example, living animal tissue is largely composed of water molecules containing hydrogen nuclei or protons. When such tissue is immersed in a magnetic field, some of the protons align with the direction of the field. This alignment may also occur for certain inanimate objects, for example, petroleum deposits in rock samples. A transient magnetic field takes the place of (or can augment) the static magnetic field (that is typically employed in conventional MRI systems).
In PRMI, following the application of the transient magnetic field, a set of magnetic gradient and/or radio-frequency pulses are typically applied, in order to read out the location and/or state of the spins. For example, a radio frequency transmitter may be used to provide an electromagnetic field whereby photons of this field having resonance frequency, flip the spin of the aligned protons. After the transient magnetic field is turned off or reduced in magnitude, the protons decay to the original spin-down state and the difference in energy between the two states is released as a photon. These photons produce a signal which can be detected by a scanner.
It is conventionally known that application of a high transient magnetic field during the polarization portion of the pulse sequence results in an improved signal (see for example, A Macovski, S Conolly: “Novel Approaches to Low-Cost MRI”, in Magnetic Resonance in Medicine 30:221-230, the subject matter of which is incorporated herein by reference in its entirety) because more spins are aligned; as a result, the application of this field subsequently results in output of a more significant signal as they return to their equilibrium state.
Reducing the fall-time of the transient magnetic field may be also advantageous, because the magnetization (due to the aligned polarized spins) does not have a chance to decay much before the readout sequence is completed. Speed of the overall pulse sequence is important in order to reduce overall scan time.
Furthermore, reducing the overall scan time may be desirable for economic reasons (for example, in order to study more patients in a fixed period of time) and/or physiological considerations (for example, to reduce the effect of breathing or cardiac motion).
The principle of PMRI has been applied to anatomical studies, as well as to explosives detection (see, for example, M Espy, M Flynn, J Gomez, C Hanson, R Kraus, P Magnelind, K Maskaly, A Matlashov, S Newman, T Owens, M Peters, H Sandin, I Savukov, L Schultz, A Urbaitis, P Volegoc, V Zotev: in “Ultra-Low Field MRI for the Detection of Liquid Explosives Using SQUIDs”, published in IEEE/CSC & ESAS European Superconductivity New Forum 8:1-12 (2009), the subject matter of which is incorporated herein by reference in its entirety).
Additionally, a variation of the pre-polarizing principle is denoted as field-cycling MRI, in which the magnitude of the transient magnetic field is varied, in order to provide information about magnetic decay properties of the object of interest (see, for example, K M Gilbert, W B Handler, T J Scholl, J W Odegaard, B A Chronik, in “Design of field-cycled magnetic resonance systems for small animal imaging”, published in Physics of Medicine and Biology 51:2825-2841 (2006) the subject matter of which is incorporated herein by reference in its entirety).
Furthermore, PMRI systems have been proposed as methods of examining neuronal activity in vivo (see, for example, R S Wijesinghe and B J Roth, in “Detection of Peripheral Nerve and Skeletal Muscle Action Currents Using Magnetic Resonance Imaging”, published in the Annals of Biomedical Engineering 37(11):2402-2406 (2009), the subject matter of which is incorporated herein by reference in its entirety).